I'm not an audio expert but but have done my fair share of tinkering. My current box is a really odd shape and I have not had any ill effects. There are plenty of boxes that aren't just "boxes" that perform well everyday. I think you would be safe with an upside down wedge as long as its all properly sealed.
Here is my box. you can see what I mean by "odd":
'14 Tuxedo Black ST3
'13 Ingot Silver Escape Titanium AWD
'04 CD Silver SVT, #877, 1 of 135 May 2011 ROTM, FF11: Best Display, Best of Show; AFN12: Best SVT, Best Audio, Finest Five, Carlisle Nifty Fifty